A. Field of the Invention
The invention relates to a drum unit and an earth plate used in a drum unit which are employed in an image forming apparatuses, such as copying machines, laser printers and facsimile machines.
B. Description of the Related Art
In an image forming apparatus, for example a copying machine, an original image or document is usually read by an exposure section to form an electrostatic latent image on a photosensitive drum. A developing unit for forming toner image is disposed about an outer circumference of the photosensitive drum. The developing unit develops the read image by charging toner supplied from a toner hopper so that it has an electrostatic charge that is opposite that of the electrostatic latent image on the photosensitive drum. The toner adheres to the photosensitive drum at oppositely charged portions of the drum corresponding to the electrostatic latent image through a developing sleeve.
The photosensitive drum includes a drum tube which is obtained by forming a conductive metal into a cylindrical shape. A flange member is forced into openings at opposing ends of the drum tube to provide bearing sections about which the drum may rotate. A support hole is provided in the center of the flange member. A shaft pin for supporting the drum unit is inserted in the support hole.
To produce a conductive state between the drum tube and the apparatus body, an earth plate may be attached to the flange member. The earth plate has a radial outer peripheral contact portion that is configured to contact the inner peripheral surface of the drum tube. An inner peripheral surface of the earth plate is configured to be in contact with the periphery of the shaft pin. The drum tube may be made of aluminum or the like and coated with an oxidized insulating film, in order to prevent corrosion. To produce conductive state between the earth plate and the drum tube, it is therefore necessary to peel part of the oxidized insulating film, which presents contact area with the peripheral contact portion of the earth plate.
As the means to improve the earth plate contruction, there has been proposed the following construction. That is, an earth plate in which a peripheral contact portion has an outer diameter which is larger than the inside diameter of a drum tube. Such an earth plate is attached to an inside surface of a flange member. When the flange member is attached to the drum tube, the peripheral contact portion of the earth plate is forced in while it scratches the inner peripheral surface of the drum tube. A portion of the earth plate is bent toward an inner radial portion of the tube such that the bent portion contacts a shaft pin. As a drum unit rotates, the peripheral contact portion remains in sliding contact with the peripheral surface of the shaft pin, thereby producing conductive state between the earth plate and the shaft pin.
In the drum unit so constructed, the conductive state between the earth plate and the drum tube is produced by forcing the flange member attached to the earth plate into the openings of the drum tube. Also, the conductive state between the earth plate and the shaft pin is produced by fitting the support hole of the flange member in the shaft pin of the image forming apparatus so that the inner peripheral contact portion of the earth plate engages the shaft pin. At this time, the peripheral contact portion is forced in together with the flange member while it scratches the inner peripheral surface of the drum tube, and therefore, the earth plate receives the force in the direction in which the center of the earth plate expands inward the drum tube. Hence, the inner peripheral contact piece of the earth plate is provided so as to project from the opposite surface so as to be in contact with the flange member of the earth plate. As the earth plate is forced in the drum tube, its end receives the force in the direction away from the shaft pin. If the earth plate is deformed under such a force, it is liable to cause contact failure between the inner peripheral contact piece and the shaft pin.
With a single inner peripheral contact portion, the contact portion presses the shaft pin in one direction. As a result, a looseness between the shaft pin and the flange member may increase deflection in the rotation of the drum unit, and may cause contact failure between the shaft pin and the inner peripheral contact piece. To avoid these problems, if the pressure intensity of the inner peripheral contact piece to the shaft pin is increased, it may cause a rotational failure of the drum unit as well as a noticeable wear of the shaft pin.